Weight-Optimized Portable Oxygen Concentrator

ABSTRACT

System for producing an oxygen-rich gas comprising (a) a primary gas mover including a first and a second compressor, wherein the primary gas mover is characterized by a weight W p ; (b) a drive motor adapted to drive the first and second compressors; (c) a rechargeable power supply characterized by a weight, W b ; and (d) a pressure/vacuum swing adsorption unit adapted to separate the pressurized feed air into an oxygen-rich product at a product flow rate F p  and an oxygen-depleted waste gas, wherein the adsorption unit comprises a plurality of adsorber beds containing an adsorbent and characterized by a total adsorbent weight W a ; and wherein the combined weight, W t , of the adsorbent, the primary gas mover, and the rechargeable power supply is characterized by the expression 
 
0.75 F p &lt;W t &lt;2.02 F p  
 
where F p  is in liters per min (at 23° C. and 1 atma pressure), and W a , W p , and W b  are in pounds.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 10/851,858, filed May 21,2004, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The supply of therapeutic oxygen to patients in homes and otherresidential settings is an important and growing market in the healthcare industry. A segment of this market includes the development andcommercialization of portable oxygen concentrators, particularly unitsthat can be carried easily by patients requiring continuous oxygentherapy. A portable and easily-carried oxygen supply may be provided bystored liquid or compressed oxygen with an appropriate vaporization orpressure regulation system and a gas delivery cannula. Alternatively andpreferably, oxygen may be supplied by a small air separation devicecarried by the patient that supplies gaseous oxygen at the desiredpurity, flow rate, and pressure. Power for operating the device can beprovided by a rechargeable power supply, typically a rechargeablebattery. The small air separation device may be an adsorption-basedsystem using a pressure swing adsorption (PSA) process.

Respiratory oxygen usage rates typically range up to about 5 lpm (litersper minute at 23° C. and 1 atma pressure) for ambulatory patients withmoderate oxygen requirements. The design of an easily-carried,rechargeable, portable oxygen concentrator in this product range shouldachieve an appropriate balance among product gas flow rate, weight, andpower supply life or run time (i.e., the operating time between powersupply recharges). This balance requires the proper choice of numerousoperating and design parameters and presents a significant challenge toengineering designers. In a small adsorptive air separation unit, forexample, design parameters may include product purity, product deliverypressure, type of process cycle, process cycle pressure envelope,adsorbent, number and dimensions of adsorbent beds, type of gas mover,type of power supply, gas flow control methods, electrical controlsystems, and materials of construction.

There is a need in the art for methods to design portableadsorption-based oxygen generation systems that provide the required gassupply rates and run times with minimum system weight. This need can bemet by optimization methods that enable designers to balance theserequirements while specifying appropriate process and mechanicalparameters for these systems.

BRIEF SUMMARY OF THE INVENTION

This need for optimized design of small, easily-carried,adsorption-based oxygen concentrators is met by the various embodimentsof the present invention. As described in detail herein, it has beenfound that a minimum weight range can be determined for anadsorption-based system for any operable combination of product flowrate, product purity, product delivery pressure, and run time. This maybe achieved by determining the weight of each variable-weight systemcomponent as a function of a selected process parameter, adding theweights of these components at various values of the selected parameter,and generating a curve of variable weight vs. the selected parameter.This curve generally exhibits a minimum weight in a preferred range ofthe selected process parameter. The selected process parameter is theminimum bed pressure during the process cycle.

An embodiment of the invention relates to a system for producing anoxygen-rich gas comprising

-   -   (a) a primary gas mover including a first compressor adapted to        compress atmospheric air to provide pressurized feed air and a        second compressor adapted to compress a waste gas from        subatmospheric pressure to atmospheric pressure, wherein the        primary gas mover is characterized by a weight W_(p);    -   (b) a drive motor adapted to drive the first and second        compressors;    -   (c) a rechargeable power supply adapted to supply power to the        drive motor, wherein the rechargeable power supply is        characterized by a weight W_(b); and    -   (d) a pressure/vacuum swing adsorption unit adapted to separate        the pressurized feed air into an oxygen-rich product at a        product flow rate F_(p) and an oxygen-depleted waste gas,        wherein the adsorption unit comprises a plurality of adsorber        beds containing an adsorbent, wherein the total amount of the        adsorbent contained in the adsorber beds is characterized by a        total adsorbent weight W_(a);        wherein the combined weight, W_(t), of the adsorbent, the        primary gas mover, and the rechargeable power supply may be        characterized by the expression        0.75 F_(p)<W_(t)<2.02 F_(p)        where F_(p) is in liters per min (at 23° C. and 1 atma pressure)        and W_(a), W_(p), and W_(b) are in pounds.

The battery may be characterized by an operating run time in hours,t_(r), between maximum and minimum working charge, and the system may befurther characterized by any of the expressions0.21 F_(p)<W_(a)<0.61 F_(p),0.36 F_(p)<W_(p)<0.70 F_(p),0.18 F_(p)<W_(b)<0.71 F_(p), and0.10 F_(p) t_(r)<W_(b)<0.40 F_(p) t_(r).

The plurality of adsorber beds may comprise four or more beds, and mayconsist of four beds.

Each of the first and second compressors may be selected from the groupconsisting of scroll, diaphragm, piston, and rotary vane compressors.The first and second compressors may be scroll-type compressors. Thesystem may further comprise a conserver.

The system may have a total weight of less than 12 pounds, may have atotal weight of less than 10 pounds, and may have a total weight of lessthan 8 pounds.

The adsorbent may be selected from the group consisting of zeolite Xexchanged with one or more metallic cations selected from the groupconsisting of lithium, calcium, zinc, copper, sodium, potassium, andsilver. The adsorber beds may further comprise an additional adsorbentselective for the adsorption of water and carbon dioxide from air andwherein the additional adsorbent is selected from the group consistingof (1) activated alumina and (2) zeolite X exchanged with one or moremetallic cations selected from the group consisting of lithium, sodium,and potassium.

The rechargeable power supply may be a battery. Alternatively, therechargeable power supply may be a fuel cell.

The system may further comprise an external case surrounding the primarygas mover, drive motor, rechargeable power supply, and pressure/vacuumswing adsorption system, and a user display/control panel mounted on theouter side of the case. This system may have a total weight of less than12 pounds, may have a total weight of less than 10 pounds, and may havea total weight of less than 8 pounds.

The system for producing an oxygen-rich gas may comprise

-   -   (a) a primary gas mover including a first compressor adapted to        compress atmospheric air to provide pressurized feed air and a        second compressor adapted to compress a waste gas from        subatmospheric pressure to atmospheric pressure, wherein the        primary gas mover is characterized by a weight W_(p);    -   (b) a drive motor adapted to drive the first and second        compressors;    -   (c) a rechargeable power supply adapted to supply power to the        drive motor, wherein the rechargeable power supply is        characterized by a weight, W_(b), and an operating run time,        t_(r), between maximum and minimum working charge; and    -   (d) a pressure/vacuum swing adsorption unit adapted to separate        the pressurized feed air into an oxygen-rich product at a        product flow rate F_(p) and an oxygen-depleted waste gas,        wherein the adsorption unit comprises a plurality of adsorber        beds containing adsorbent, wherein the total amount of the        adsorbent contained in the adsorber beds is characterized by a        total adsorbent weight W_(a);        wherein the system may be characterized by any of the        expressions        0.21 F_(p)<W_(a)<0.61 F_(p),        0.36 F_(p)<W_(p)<0.70 F_(p),        0.18 F_(p)<W_(b)<0.71 F_(p), and        0.10 F_(p) t_(r)<W_(b)<0.40 F_(p) t_(r),        where F_(p) is in liters per min (at 23° C. and 1 atma        pressure), t_(r) is in hours, and W_(a), W_(p) and W_(b), are in        pounds.

The system may further comprise additional elements including electricalwiring and control systems, a case or housing, and a userdisplay/control panel mounted on the outer side of the housing, whereinthe oxygen generation system and the additional elements are combined toform a portable oxygen concentrator, and means for the user to carry theportable concentrator unit.

Another embodiment of the invention pertains to a method for producingan oxygen-rich product gas comprising

-   -   (a) providing a primary gas mover including a first compressor        for compressing atmospheric air to provide pressurized feed air        and a second compressor adapted to compress an oxygen-depleted        waste gas from subatmospheric pressure to atmospheric pressure,        a drive motor for driving the first and second compressors, and        a rechargeable battery for providing power to the drive motor,        wherein the rechargeable power supply is characterized by an        operating run time between maximum and minimum working charge;    -   (b) providing a pressure/vacuum swing adsorption system adapted        to separate the pressurized feed air into the oxygen-rich        product gas and the oxygen-depleted waste gas, wherein the        adsorption system comprises a plurality of adsorber beds        containing adsorbent; and    -   (c) operating each of the adsorber beds in turn through an        adsorption cycle including at least the repeating steps of        feed/provide product, depressurization, evacuation, and        repressurization;        wherein the method may be characterized by any of the operating        parameters    -   (1) the rechargeable battery provides between 0.02 and 0.17 KWh        of power during the operating run time between maximum and        minimum working charge;    -   (2) the total working capacity of the adsorbent in each adsorber        bed during the adsorption cycle is between 1.2×10⁻⁴ and 6.7×10⁻⁴        lbmoles of nitrogen;    -   (3) the first compressor moves between 1.14×10⁻⁴ and 4.01×10⁻⁴        lbmoles of pressurized feed air during the feed/provide product        step; and    -   (4) the second compressor moves between 3.47×10⁻⁴ and 9.96×10⁻⁴        lbmoles of waste gas during the depressurization and evacuation        steps.

The pressure/vacuum swing adsorption system may have four adsorber bedsand each of the adsorber beds may undergo in turn a series of adsorptioncycle steps which comprise

-   -   (A) a feed/make product step wherein the pressurized feed air is        introduced into a feed end of the bed while the oxygen-enriched        product gas is withdrawn from a product end of the bed;    -   (B) a feed/make product/provide repressurization step wherein        the pressurized feed air is introduced into a feed end of the        bed while an oxygen-enriched product gas is withdrawn from a        product end of the bed, and wherein a portion of the product gas        is used for pressurizing another bed undergoing its final        repressurization step;    -   (C) a depressurization step in which the bed is depressurized by        withdrawing gas therefrom, wherein at least a portion of the gas        withdrawn therefrom is transferred to another bed undergoing a        repressurization step;    -   (D) a provide purge step in which the bed is further        depressurized by withdrawing gas therefrom, wherein at least a        portion of the gas withdrawn therefrom is transferred to another        bed undergoing a purge step;    -   (E) an evacuation step in which gas is withdrawn from the feed        end of the bed until the bed reaches a minimum subatmospheric        bed pressure;    -   (F) a purge step in which the bed is purged by introducing purge        gas into the product end of the bed while continuing to evacuate        the bed, wherein the purge gas is provided from another bed        undergoing step (D);    -   (G) a repressurization step in which pressurization gas is        introduced into the product end of the bed, wherein the        pressurization gas is provided from another bed undergoing step        (C); and    -   (H) a final repressurization step in which product gas from        another bed is introduced into the product end of the bed.

The minimum bed pressure may be between 0.25 and 1.0 atma, and may bebetween 0.45 and 0.8 atma. The pressure of the oxygen-enriched productgas may be between 1.2 and 1.6 atma. The oxygen-enriched product gas maybe provided at a flow rate in the range of 0.5 to 3.0 liters per min(defined at 23° C. and 1 atma pressure).

An alternative embodiment of the invention is directed to a method forproducing an oxygen-rich product gas comprising

-   -   (a) providing a primary gas mover including a first compressor        for compressing atmospheric air to provide pressurized feed air        and a second compressor adapted to compress an oxygen-depleted        waste gas from subatmospheric pressure to atmospheric pressure,        a drive motor for driving the first and second compressors, and        a rechargeable battery for providing power to the drive motor,        wherein the rechargeable power supply is characterized by an        operating run time between maximum and minimum working charge;    -   (b) providing a pressure/vacuum swing adsorption unit adapted to        separate the pressurized feed air into the oxygen-rich product        gas and the oxygen-depleted waste gas, wherein the adsorption        unit comprises a plurality of adsorber beds containing adsorbent        selective for the adsorption of nitrogen from air; and    -   (c) operating each of the adsorber beds in turn through an        adsorption cycle including at least the repeating steps of        feed/provide product, depressurization, evacuation, and        repressurization;        wherein the minimum pressure in the evacuation step may be        between 0.35 and 1.00 atma.

Another embodiment of the invention relates to a method for the designof a portable pressure/vacuum swing adsorption oxygen concentratorsystem comprising

-   -   (a) defining design parameters including at least a product flow        rate, a product purity, a product delivery pressure, a        pressure/vacuum swing adsorption process cycle, the number of        adsorber vessels, an adsorbent contained in the adsorber        vessels, the type of gas mover, the type of regenerable power        supply to provide power to the drive motor, and the run time of        the regenerable power supply between maximum and minimum working        charge;    -   (b) selecting a series of minimum bed pressures pressures below        atmospheric pressure and determining for each of the minimum bed        pressures the required weights of the gas mover, the power        supply, and the adsorbent contained in the adsorber vessels,        wherein each minimum bed pressure is a lowest bed pressure in        the pressure/vacuum swing adsorption process cycle;    -   (c) adding the weights of the adsorbent, the gas mover, and the        power supply determined in (b) for each value of the minimum bed        pressure to provide a total weight of the adsorbent, the gas        mover, and the power supply as a function of the minimum bed        pressure; and    -   (d) selecting a range of the minimum bed pressures that        corresponds to a range of minimum combined weight of the        adsorbent, the gas mover, and the power supply.        The range of minimum bed pressures may be between 0.45 and 0.8        atma.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of an exemplary pressure/vacuum swingadsorption system for embodiments of the present invention.

FIG. 2 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a five bed PVSAsystem illustrating a first embodiment of the invention.

FIG. 3 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a five bed PVSAsystem illustrating a second embodiment of the invention.

FIG. 4 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a five bed PVSAsystem illustrating a third embodiment of the invention.

FIG. 5 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a five bed PVSAsystem illustrating a fourth embodiment of the invention.

FIG. 6 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a five bed PVSAsystem illustrating a fifth embodiment of the invention.

FIG. 7 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a five bed PVSAsystem illustrating a sixth embodiment of the invention.

FIG. 8 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a five bed PVSAsystem illustrating a seventh embodiment of the invention.

FIG. 9 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a five bed PVSAsystem illustrating an eighth embodiment of the invention.

FIG. 10 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a four bed PVSAsystem illustrating a first alternative embodiment of the invention.

FIG. 11 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a four bed PVSAsystem illustrating a second alternative embodiment of the invention.

FIG. 12 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a four bed PVSAsystem illustrating a third alternative embodiment of the invention.

FIG. 13 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a four bed PVSAsystem illustrating a fourth alternative embodiment of the invention.

FIG. 14 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a four bed PVSAsystem illustrating a fifth alternative embodiment of the invention.

FIG. 15 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a four bed PVSAsystem illustrating a sixth alternative embodiment of the invention.

FIG. 16 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a four bed PVSAsystem illustrating a seventh alternative embodiment of the invention.

FIG. 17 is a plot of individual component variable weights and totalcomponent variable weight vs. minimum bed pressure for a four bed PVSAsystem illustrating an eighth alternative embodiment of the invention.

FIG. 18 is a plot of adsorbent weights vs. product flow rate forExamples 1-16.

FIG. 19 is a plot of the weights of the primary gas mover vs. productflow rate for Examples 1-16.

FIG. 20 is a plot of the battery weights vs. product flow rate forExamples 1-16.

FIG. 21 is a plot of the time-normalized battery weights vs. productflow rate for Examples 1-16.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the invention described herein are directed tomethods for designing and optimizing the weight of small pressure/vacuumswing adsorption (PVSA) systems utilized, for example, in portable anduser-carried medical oxygen concentrator systems. It was found in thedevelopment of the embodiments of the present invention that a minimumweight or desirable range of weights can be determined for the PVSAsystem for any operable combination of product flow rate, productpurity, product delivery pressure, and run time. This may be achieved bydetermining the weight of each variable-weight system component as afunction of a selected process parameter, adding the weights of thesecomponents at various values of the selected parameter, and generating acurve of variable weight vs. the selected parameter. This curvegenerally exhibits a desirable minimum weight or range of minimumweights as a function of the selected process parameter. This selectedprocess parameter may be the minimum bed pressure during regeneration inthe PVSA cycle.

In the PVSA process described herein, the adsorber bed pressures varybetween superatmospheric pressure and subatmospheric pressure duringeach cycle as described below. This differs from a pressure swingadsorption (PSA) process in which the operating pressure range includesbed pressures above atmospheric pressure and may include bed pressuresapproaching atmospheric pressure at the end of the depressurizationstep. Subatmospheric pressures are not utilized in a PSA process.

An exemplary PVSA process and system that may be designed according toembodiments of the invention is shown for the purpose of illustration inFIG. 1. Atmospheric air 1 is drawn through filter 3, inlet silencer 5,and line 7 by first or feed air compressor 9. Feed air compressor 9 is apart of primary gas mover 11 which also includes drive motor 13 andsecond or vacuum waste gas compressor 15. Pressurized feed air at 1.15to 1.80 atma is discharged from the compressor and flows through airfeed line 17 to rotary valve assembly 19, which is in flow communicationwith adsorber bed feed lines 21, 23, 25, 27, and 29, adsorber bedproduct lines 31, 33, 35, 37, and 39, air feed line 17, product line 51,and waste gas line 53. In this exemplary PVSA system, five adsorber beds41, 43, 45, 47, and 49 are used, although any number of multiple bedsmay be used. An optional product gas storage tank (not shown) may beused if desired. A cannula (not shown) may be connected to product line51 to deliver product gas to the user.

Each adsorber bed contains adsorbent selective for the adsorption ofwater, carbon dioxide, and nitrogen from air. This adsorbent may beselected from the group consisting of zeolite X exchanged with one ormore metallic cations selected from the group consisting of lithium,calcium, zinc, copper, sodium, potassium, and silver. The zeolite X mayhave a ratio of silicon to aluminum of about 1 to about 1.25. Theadsorbent may be formed into beads, extrudates, or other shapes known inthe art, using binder materials or without binder materials (also knownas binderless). The adsorbent typically adsorbs water and carbon dioxidemore strongly than nitrogen, and therefore the initial adsorbentadjacent to the feed air inlet of an adsorber will preferentially removewater and carbon dioxide. Dry, carbon dioxide-free air from this initialadsorbent region then passes to the remainder of the adsorbent in theadsorber, where the nitrogen is selectively adsorbed to provide theoxygen-enriched product gas. The initial adsorbent adjacent to the feedair inlet thereby provides pretreatment by removing water and carbondioxide prior to nitrogen removal.

Optionally, each adsorber bed also may contain pretreatment adsorbentselective for the adsorption of water and carbon dioxide from air, andthis adsorbent may be selected from the group consisting of (1)activated alumina and (2) zeolite X exchanged with one or more metalliccations selected from the group consisting of lithium, sodium, andpotassium. Typically, the water-selective adsorbent (if used) would forma layer located adjacent the feed end of the adsorber bed and maycomprise 10 to 40% of the total adsorbent in the adsorber bed. In thisoption, the remainder of the bed would contain the adsorbent describedabove and would selectively adsorb nitrogen from the water and carbondioxide-free air from the pretreatment adsorbent layer.

Vacuum waste gas compressor 15 withdraws oxygen-depleted PVSA waste gasthrough line 53, typically at subatmospheric pressure, and dischargesthe gas via line 57 and silencer 57 to the atmosphere. Electric powerfor drive motor 13 is provided by rechargeable power supply 59, whichmay be a rechargeable battery of any type known in the art.Alternatively, the rechargeable power supply may be a portable fuel cellsystem comprising a fuel cell and portable fuel storage means. The fuelmay be hydrogen or methanol.

Feed air compressor 9 and vacuum waste gas compressor 15 may be any typeof compressor known in the art and may be selected from scroll,diaphragm, piston, and rotary vane compressors. The feed air and vacuumwaste gas compressors may be driven in tandem by a single drive motorand may be driven by a common drive shaft. Scroll compressors arewell-suited for service with the air separation device described herein.Feed air compressor 9 and vacuum waste gas compressor 15 may be combinedin a single combined scroll-type primary gas mover.

Rotary valve assembly 19 is designed for a specific PVSA cycle and aspecific number of adsorber beds. The assembly includes a first rotaryvalve connected to lines 21, 23, 25, 27, and 29 that are attached to thefeed ends of adsorbent beds 41, 43, 45, 47, and 49, respectively. Thefirst rotary valve also is connected to air feed line 17 and waste gasline 53. This first rotary valve enables appropriate flow communicationamong any of the feed ends of the adsorbent beds, the air feed line, andthe waste gas line according to predetermined process cycle steps asdescribed below. A second rotary valve is connected to lines 31, 33, 35,37, and 39 that are attached to the product ends of the adsorbent beds,respectively, and also is connected to product line 51. This secondrotary valve enables appropriate flow communication among any of theproduct ends of the adsorbent beds and the product line according topredetermined process cycle steps as described below. The two rotaryvalves may be operated by a single drive motor and may rotate at thesame rotational rate. Rotary valves of this type are described, forexample, in a copending U.S. patent application having Ser. No.10/295,144 filed on Nov. 15, 2002. This patent application isincorporated herein by reference.

The operation of the PVSA system of FIG. 1 may be illustrated by anexemplary PVSA cycle summarized in Table 1. TABLE 1 Process Cycle Stepsfor Exemplary PVSA System Step Duration, Number Description Sec. 1Feed/make product/provide purge 1.0 2 Feed/make product/provide repress.gas 1.0 3 provide 1^(st) repressurization gas 1.0 4 provide 2^(nd)repressurization gas 1.0 5 Idle step 1.0 6 Evacuation 1.0 7 Purge withproduct gas 1.0 8 Receive 2^(nd) repressurization gas 1.0 9 Receive1^(st) repressurization gas 1.0 10 Repressurize with product gas 1.0

During the initial portion of Step 1, the feed step, there may be ashort period of feed pressurization before product gas flows from thebed. During evacuation in Step 6, a minimum bed pressure is attained,which is defined as the lowest pressure during this step. The durationof this exemplary 10 second cycle, or the duration of any step in thecycle, may be modified as required to meet various process or productrequirements.

A cycle chart is given in Table 2 to show the relationship of cyclesteps among the five adsorbent beds, wherein each bed in turn passesthrough steps 1-10 of Table 1. TABLE 2 PVSA Cycle Chart Bed Step Number41 1 2 3 4 5 6 7 8 9 10 43 9 10 1 2 3 4 5 6 7 8 45 7 8 9 10 1 2 3 4 5 647 5 6 7 8 9 10 1 2 3 4 49 3 4 5 6 7 8 9 10 1 2

The use of Tables 1 and 2 together with FIG. 1 will enable the skilledperson to understand this exemplary PVSA process cycle. Modifications tothis particular cycle may be made if desired, and other types of PVSAcycles may be used as appropriate.

As an alternative embodiment to the five bed PVSA system and cycledescribed above, a four bed system and cycle may be used. This four bedsystem would be a modification of the system of FIG. 1 wherein adsorberbed 49, adsorber bed feed line 29, and adsorber product line 39 aredeleted. Rotary valve 19 would be designed for four beds instead of fivebeds. In this alternative cycle, only one pressurization gas transferstep is used compared with two such steps in the five bed cycle ofTable 1. Table 3 presents the four bed cycle steps and Table 4 presentsa cycle chart for the four bed cycle (note that bed 49 of FIG. 1 isdeleted for the four bed system). TABLE 3 Process Cycle Steps forExemplary 4-Bed PVSA System Step Duration, Number Description Sec. 1Feed/make product 1.0 2 Feed/make product/provide repress. gas 1.0 3Provide repressurization gas 1.0 4 Provide purge 1.0 5 Evacuation 1.0 6Purge 1.0 7 Receive repressurization gas 1.0 8 Repressurize with productgas 1.0

TABLE 4 4-Bed PVSA Cycle Chart Bed Step Number 41 1 2 3 4 5 6 7 8 43 7 81 2 3 4 5 6 45 5 6 7 8 1 2 3 4 47 3 4 5 6 7 8 1 2

A complete portable user-carried oxygen concentrator system typicallyincludes a number of components in addition to those illustrated by theexemplary PVSA system of FIG. 1. These additional components mayinclude, for example, any of the following features: electrical wiringand control systems;; structural elements; a case or housing; a userdisplay/control panel mounted on the outer side of the housing; aconserver; a product tank; and means for the user to carry theconcentrator unit such as a handle, carrying strap, or dual shoulderstraps. The total weight of the portable user-carried oxygenconcentrator system thus is the combined weight of (a) thevariable-weight components earlier described (i. e., the adsorbent,primary gas mover, and the battery) and (b) the additional componentsdescribed immediately above.

Portable user-carried oxygen concentrator systems such as that thosedescribed above using four or five beds may be designed to meetdesirable criteria such as, for example, a continuous oxygen productflow of up to 3 lpm, an easily-carried weight, and an operating time ona single power supply recharge of at least 1-2 hours. A system meetingthese criteria would provide more freedom and a higher standard ofliving for an ambulatory patient and would be an attractive productoffering for a supplier of oxygen concentrators.

Embodiments of the PVSA oxygen concentrator system described abovepreferably meet these criteria and provide the patient with anoxygen-enriched product of at least 85 mole % oxygen purity. Theportable oxygen concentrator system should be easily carried by thepatient and have a total weight of less than 12 pounds, preferably lessthan 10 pounds, and most preferably less than 8 pounds. Because patientsneeding oxygen therapy usually are ill, minimum system weight isextremely important. As mentioned earlier, designing these systems forminimum weight is a significant engineering challenge.

When the product flow rate, product purity, product delivery pressure,and system run time are specified, the total weight of the oxygenconcentrator system consists of some components whose weights depend onthe PVSA operating conditions and other components whose weights areessentially independent of PVSA operating conditions. Thevariable-weight components in this scenario include the power supply(e.g., battery), the weight of adsorbent in the adsorbent beds, and theweight of the primary gas mover, i.e., the feed air compressor andvacuum waste gas compressor in combination. The weights of all othercomponents in this scenario are independent of the selection of productflow rate, product purity, product delivery pressure, and system runtime. The total weight of the oxygen concentrator system thus may beminimized by selecting PVSA operating conditions that minimize theweight of the variable-weight components.

The embodiments of the present invention are directed to methods forminimizing the weight of the variable-weight components by optimizingPVSA operating conditions as described below. Reducing the weight of thefixed-weight components (i.e., those components whose weights areessentially independent of PVSA cycle operating conditions) may bepossible by improvements in materials, motor design, electrical systems,and the like, but these are not addressed by embodiments of the presentinvention.

The adsorbent weight requirement may be determined by the amount ofadsorbent required to remove the nitrogen from feed air such that adesired oxygen product purity is attained. The adsorbent weightrequirement can be determined by the relation$W_{a} = {\frac{n_{ads}}{n}*\frac{1}{B}}$where n_(ads) is the moles of nitrogen to be removed per minute, n isthe nitrogen working capacity in moles of nitrogen adsorbed by theadsorbent in one adsorber bed during one bed cycle, and B is the rate atwhich a fresh adsorber bed is available for feed in beds/min and isdetermined by the PVSA cycle time. The parameter n_(ads) can bedetermined by:$n_{ads} = \frac{Q_{p}y_{{O\quad 2},p}y_{{N\quad 2},f}}{\theta_{O\quad 2}y_{{O\quad 2},f}}$where Q_(p) is the product flow in moles per minute, y_(O2,p) is theproduct purity in percent oxygen, y_(N2,f) is the nitrogen concentrationin the feed in percent, θ_(O2) is the oxygen recovery in per cent (i.e.,the percent of the oxygen in the feed gas that is present in the productgas), and y_(O2,f) is the oxygen concentration in the feed in percent.The nitrogen working capacity of the adsorbent is dependent on thepressure envelope to which the adsorbent is exposed. The preferredmethod to determine adsorbent working capacity is to measure oxygen andnitrogen pure component isotherms at multiple temperatures from whichparameters can be determined by the application of the dual siteLangmuir model [see Myers, A. L., Activity Coefficients of MixturesAdsorbed on Heterogeneous Surfaces, AlChE J. 1983 (29), 691]$n_{i} = {\frac{M_{1}{bp}}{1 + {bp}} + \frac{M_{2}{dp}}{1 + {dp}}}$where M₁, b, M₂, and d are fit parameters and p is pressure. TheLangmuir model then is used to determine working capacity by means ofmulticomponent adsorption models, namely the ideal adsorption solutiontheory (IAST) [see Myers, A. L. and Prausnitz, J. M., Thermodynamics ofMixed Gas Adsorption, AlChE J. 1965 (1), 11] or more preferably theheterogeneous ideal adsorbed solution theory (HIAST) [see Mathias P. M.et al, Correlation of Multicomponent Gas Adsorption by the Dual-SiteLangmuir Model. Application to Nitrogen/Oxygen Adsorption on 5A Zeolite,Ind. & Eng. Chem Res. 1996 (35), 7].

The weight of the primary gas mover (i.e., the combined weight of thefeed air compressor and the vacuum waste gas compressor), W_(p), may bedetermined based on requirements of the two compressors to provide gasat the specified pressures during the feed step of the cycle and therequired flow rate during the vacuum and purge steps of the cycle. Theweight of the primary gas mover will vary based on the geometry of thecompressor; for example, the size of the involutes in a scrollcompressor will vary based on the gas compression ratio. The weight ofthe primary gas mover does not include the electric motor which powersthe primary gas mover and is considered a fixed weight for the presentanalysis, wherein the motor can be operated at various speeds dependingon the required feed gas and waste gas compression duty. The weight ofthe primary gas mover is determined to be proportional to the oxygenproduction rate for the present analysis. The weight of the rechargeablepower supply, in this case a battery, may be optimized by applying therelationship of energy discharge to the requirements over the durationof the PVSA cycle. The power supplied by the battery to the othercomponents of the oxygen generator (alarms, valve motor, etc) may beabout 5 W. The power required from the battery to operate the feed aircompressor and the vacuum waste gas compressor may be determineddirectly by the adiabatic power of compression based on the pressuresused during the PVSA cycles. Adiabatic power is given by the expression$P_{ad} = {\frac{\overset{.}{m}{kRT}_{1}}{k - 1}\left\lbrack {\left( \frac{p_{2}}{p_{1}} \right)^{(\frac{k - 1}{k})} - 1} \right\rbrack}$where {dot over (m)} is the mass flow rate, R is the gas constant, T₁ isthe temperature of the inlet gas, p₂ is the pressure of the outlet gas,p₁ is the pressure of the inlet gas to the compressor, and k is theratio of heat capacity at constant pressure to heat capacity at constantvolume and equals 1.4 for air. When operating in the compression mode,p₂ is the air feed pressure and p₁ is atmospheric pressure. Whenoperating in the vacuum mode, p₂ is atmospheric pressure and p₁ is thewaste gas pressure exiting the adsorbent bed.

The battery power density may be determined from manufacturers'specifications. For a state-of-the-art lithium ion battery, for example,the energy density ρ_(batt) is given in lb/Wh. For any given run time,t_(r) (in hours), the weight of the battery (in pounds) may be describedby the relation$W_{b} = \frac{\rho_{batt}P_{ad}t_{r}}{\eta_{p}\eta_{m}}$where η_(p) and η_(m) are the efficiencies of the compressors and thedrive motor, respectively.

The overall weight of the variable-weight components may be determinedfrom the relationship of each individual component weight to thecharacteristics of the cycle, specifically the operating pressureenvelope. The total weight of the variable-weight components thereforeis a function of the minimum pressure during evacuation, p_(min), andthe product pressure, p_(prod).

The desirable weight of the variable components may be determined byfirst selecting a product flow rate, product pressure, and run time.Then, using the total weight function, the combined weight of all threecomponents can be plotted as a function of a single variable, theminimum bed pressure, as given below:W _(t) ={W _(a) +W _(p) +W _(b)}(p _(min))

The desirable weight of the variable components is determined by firstselecting a product flow rate, product pressure, and run time. Then,using the total weight function, the combined weight of all threecomponents can be plotted as a function of a single variable, theminimum bed pressure. Plotting the weight of the variable-weightcomponents vs. the minimum bed pressure at constant production rate,product purity, product pressure, and run time shows unexpectedly thatthere is a minimum pressure, or a range of desirable minimum pressures,that correspond to a minimum weight or range of desirable minimumweights of the variable-weight components.

The following Examples illustrate this feature but do not limit theembodiments of the invention to any of the specific details describedtherein. Each of the Examples is based on providing a product containing93 mole % oxygen at various delivery pressures, flow rates, and runtimes using (a) the five bed PVSA system of FIG. 1 with the PVSA cycledescribed in Tables 1 and 2, and (b) a four bed PVSA system with thecycle described in Tables 3 and 4. The adsorbent is a sodium- andlithium-exchanged low-silica X-type zeolite (LSX) in bead form with anaverage particle diameter of 0.50 mm. In calculating the weight of theadsorbent required using the nitrogen adsorption equations given above,a bed utilization factor of 70% was used to account for the fact that70% of the adsorbent capacity is utilized for nitrogen adsorption whilethe remaining 30% of the adsorbent capacity is utilized for theadsorption of water and carbon dioxide.

The efficiency of drive motor 13 in primary gas mover 11 typically maybe 80% and the efficiency of compressors 9 and 15 typically may be 70%.The system is powered by a rechargeable lithium ion battery such as, forexample, one manufactured and sold by Varta having a fixed energydensity of 12.46 lb/kWh per the manufacturer's specifications. The totalweight of the system is the sum of the weight of the fixed-weightcomponents (housing, tubing, electrical wiring, etc) and the variableweights of the adsorbent, the primary gas mover (i.e., the feed aircompressor and the vacuum waste gas compressor), and the battery.

EXAMPLE 1

A PVSA system was simulated to generate 3 lpm of 93 mole % oxygen at aproduct pressure of 1.6 atm for a period of 1 hour of continuous runtime for a five-bed system of FIG. 1 using the cycle of Tables 1 and 2.The primary gas mover consisted of scroll-type feed air and waste gascompressors driven by a common motor. The weight of each variable-weightcomponent, i.e., the adsorbent, primary gas mover, and battery, werecalculated using the methods described earlier for values of the minimumbed pressure between 0.35 and 1.0 atma. These weights were summed andall data were plotted as shown in FIG. 2. The three individual componentweights show no obvious minima as functions of the minimum bed pressure.When these weights are combined, however, the plot of total variableweight vs. minimum bed pressure exhibits a minimum total variable weightof 3.6 lb at 0.7 atma. A desirable relative weight range between theminimum weight and 5% above the minimum weight was defined to yield adesirable total variable weight range of 3.6 to 3.8 lb. This correspondsto a range of the minimum bed pressure of 0.4 to 0.9 atma, which is adesirable PVSA operating range for this Example. In this desirablepressure range, the weight of the adsorbent is between a lower value of0.7 lb and an upper value of 1.4 lb, the weight of the primary gas moveris 1.9 lb, and the weight of the battery is between a lower value of 0.5lb and an upper value of 1.2 lb.

EXAMPLE 2

Example 1 was repeated using a primary gas mover consisting ofdiaphragm-type feed air and waste gas compressors driven by a commonmotor. The weight of each variable-weight component, i.e., theadsorbent, primary gas mover, and battery, were calculated using themethods described earlier for values of the minimum bed pressure between0.35 and 0.96 atma. These weights were summed and all data were plottedas shown in FIG. 3. The three individual component weights show noobvious minima as functions of the minimum bed pressure. When theseweights are combined, however, the plot of total variable weight vs.minimum bed pressure exhibits a minimum total variable weight of 2.8 lbat about 0.7 atma. A desirable relative weight range between the minimumweight and 5% above the minimum weight was defined to yield a desirabletotal variable weight range of 2.8 to 3.0 lb. This corresponds to arange of the minimum bed pressure of 0.5 to 0.9 atma, which is adesirable PVSA operating range for this Example. In this desirablepressure range, the weight of the adsorbent is between a lower value of0.7 lb and an upper value of 1.3 lb, the weight of the primary gas moveris 1.1 lb, and the weight of the battery is between a lower value of 0.6lb and an upper value of 1.1 lb.

EXAMPLE 3

A PVSA system was simulated to generate 2 lpm of 93 mole % oxygen at aproduct pressure of 1.4 atm for a period of 2 hours of continuous runtime for a five-bed system of FIG. 1 using the cycle of Tables 1 and 2.The primary gas mover consisted of scroll-type feed air and waste gascompressors driven by a common motor. The weight of each variable-weightcomponent, i.e., the adsorbent, primary gas mover, and battery, werecalculated using the methods described earlier for values of the minimumbed pressure between 0.4 and 1.0 atma. These weights were summed and alldata were plotted as shown in FIG. 4. The three individual componentweights show no obvious minima as functions of the minimum bed pressure.When these weights are combined, however, the plot of total variableweight vs. minimum bed pressure exhibits a minimum total variable weightof 2.9 lb at 0.7 atma. A desirable relative weight range between theminimum weight and 5% above the minimum weight was defined to yield adesirable total variable weight range of 2.9 to 3.1 lb. This correspondsto a range of the minimum bed pressure of about 0.6 to about 0.9 atma,which is a desirable PVSA operating range for this Example. In thisdesirable pressure range, the weight of the adsorbent is between a lowervalue of 0.6 lb and an upper value of 1.2 lb, the weight of the primarygas mover is 1.3 lb, and the weight of the battery is between a lowervalue of 0.5 lb and an upper value of 1.1 lb.

EXAMPLE 4

Example 3 was repeated using a primary gas mover consisting ofdiaphragm-type feed air and waste gas compressors driven by a commonmotor. The weight of each variable-weight component, i.e., theadsorbent, primary gas mover, and battery, were calculated using themethods described earlier for values of the minimum bed pressure between0.40 and 0.96 atma. These weights were summed and all data were plottedas shown in FIG. 5. The three individual component weights show noobvious minima as functions of the minimum bed pressure. When theseweights are combined, however, the plot of total variable weight vs.minimum bed pressure exhibits a minimum total variable weight of 2.3 lbat about 0.8 atma. A desirable relative weight range between the minimumweight and 5% above the minimum weight was defined to yield a desirabletotal variable weight range of 2.3 to 2.5 lb. This corresponds to arange of the minimum bed pressure of 0.6 to about 0.9 atma, which is adesirable PVSA operating range for this Example. In this desirablepressure range, the weight of the adsorbent is between a lower value of0.6 lb and an upper value of 1.1 lb, the weight of the primary gas moveris 0.7 lb, and the weight of the battery is between a lower value of 0.6lb and an upper value of 1.1 lb.

EXAMPLE 5

A PVSA system was simulated to generate 1 lpm of 93 mole % oxygen at aproduct pressure of 1.2 atm for a period of 3 hours of continuous runtime for a five-bed system of FIG. 1 using the cycle of Tables 1 and 2.The primary gas mover consisted of scroll-type feed air and waste gascompressors driven by a common motor. The weight of each variable-weightcomponent, i.e., the adsorbent, primary gas mover, and battery, werecalculated using the methods described earlier for values of the minimumbed pressure between 0.35 and 0.96 atma. These weights were summed andall data were plotted as shown in FIG. 6. The three individual componentweights show no obvious minima as functions of the minimum bed pressure.When these weights are combined, however, the plot of total variableweight vs. minimum bed pressure exhibits a minimum total variable weightof 1.5 lb at about 0.7 atma. A desirable relative weight range betweenthe minimum weight and 5% above the minimum weight was defined to yielda desirable total variable weight range of 1.5 to 1.6 lb. Thiscorresponds to a range of the minimum bed pressure of about 0.6 to 0.8atma, which is a desirable PVSA operating range for this Example. Inthis desirable pressure range, the weight of the adsorbent is between alower value of 0.3 lb and an upper value of 0.6 lb, the weight of theprimary gas mover is 0.7 lb, and the weight of the battery is between alower value of 0.3 lb and an upper value of 0.6 lb.

EXAMPLE 6

Example 5 was repeated using a primary gas mover consisting ofdiaphragm-type feed air and waste gas compressors driven by a commonmotor. The weight of each variable-weight component, i.e., theadsorbent, primary gas mover, and battery, were calculated using themethods described earlier for values of the minimum bed pressure between0.40 and 0.96 atma. These weights were summed and all data were plottedas shown in FIG. 7. The three individual component weights show noobvious minima as functions of the minimum bed pressure. When theseweights are combined, however, the plot of total variable weight vs.minimum bed pressure exhibits a minimum total variable weight of 1.3 lbat about 0.7 atma. A desirable relative weight range between the minimumweight and 5% above the minimum weight was defined to yield a desirabletotal variable weight range of 1.2 to 1.3 lb. This corresponds to arange of the minimum bed pressure of 0.6 to about 0.8 atma, which is adesirable PVSA operating range for this Example. In this desirablepressure range, the weight of the adsorbent is between a lower value of0.3 lb and an upper value of 0.6 lb, the weight of the primary gas moveris 0.4 lb, and the weight of the battery is between a lower value of 0.3lb and an upper value of 0.6 lb.

EXAMPLE 7

A PVSA system was simulated to generate 3 lpm of 93 mole % oxygen at aproduct pressure of 1.6 atm for a period of 3 hours of continuous runtime for a five-bed system of FIG. 1 using the cycle of Tables 1 and 2.The primary gas mover consisted of scroll-type feed air and waste gascompressors driven by a common motor. The weight of each variable-weightcomponent, i.e., the adsorbent, primary gas mover, and battery, werecalculated using the methods described earlier for values of the minimumbed pressure between 0.5 and 1.06 atma. These weights were summed andall data were plotted as shown in FIG. 8. The three individual componentweights show no obvious minima as functions of the minimum bed pressure.When these weights are combined, however, the plot of total variableweight vs. minimum bed pressure exhibits a minimum total variable weightof 4.8 lb at about 0.9 atma. A desirable relative weight range betweenthe minimum weight and 5% above the minimum weight was defined to yielda desirable total variable weight range of 4.8 to 5.0 lb. Thiscorresponds to a range of the minimum bed pressure of about 0.8 to 1.1atma, which is a desirable PVSA operating range for this Example. Inthis desirable pressure range, the weight of the adsorbent is between alower value of 1.0 lb and an upper value of 1.8 lb, the weight of theprimary gas mover is 1.9 lb, and the weight of the battery is between alower value of 1.4 lb and an upper value of 2.1 lb.

EXAMPLE 8

Example 7 was repeated using a primary gas mover consisting ofdiaphragm-type feed air and waste gas compressors driven by a commonmotor. The weight of each variable-weight component, i.e., theadsorbent, primary gas mover, and battery, were calculated using themethods described earlier for values of the minimum bed pressure between0.37 and 1.06 atma. These weights were summed and all data were plottedas shown in FIG. 9. The three individual component weights show noobvious minima as functions of the minimum bed pressure. When theseweights are combined, however, the plot of total variable weight vs.minimum bed pressure exhibits a minimum total variable weight of 4.0 lbat about 0.9 atma. A desirable relative weight range between the minimumweight and 5% above the minimum weight was defined to yield a desirabletotal variable weight range of 4.0 to 4.2 lb. This corresponds to arange of the minimum bed pressure of 0.8 to about 1.0 atma, which is adesirable PVSA operating range for this Example. In this desirablepressure range, the weight of the adsorbent is between a lower value of1.1 lb and an upper value of 1.6 lb, the weight of the primary gas moveris 1.1 lb, and the weight of the battery is between a lower value of 1.4lb and an upper value of 2.0 lb.

EXAMPLE 9

Example 1 was repeated except that the PVSA system was a four bed systemoperated according to the cycle of Tables 3 and 4. The primary gas moverconsisted of scroll-type feed air and waste gas compressors driven by acommon motor. The weight of each variable-weight component, i.e., theadsorbent, primary gas mover, and battery, were calculated using themethods described earlier for values of the minimum bed pressure between0.35 and 1.0 atma. These weights were summed and all data were plottedas shown in FIG. 10. The three individual component weights show noobvious minima as functions of the minimum bed pressure. When theseweights are combined, however, the plot of total variable weight vs.minimum bed pressure exhibits a minimum total variable weight of 3.2 lbat 0.7 atma. A desirable relative weight range between the minimumweight and 5% above the minimum weight was defined to yield a desirabletotal variable weight range of 3.2 to 3.4 lb. This corresponds to arange of the minimum bed pressure of 0.5 to about 0.9 atma, which is adesirable PVSA operating range for this Example. In this desirablepressure range, the weight of the adsorbent is between a lower value of0.6 lb and an upper value of 1.3 lb, the weight of the primary gas moveris 1.6 lb, and the weight of the battery is between a lower value of 0.5lb and an upper value of 1.2 lb.

EXAMPLE 10

Example 2 was repeated except that the PVSA system was a four bed systemoperated according to the cycle of Tables 3 and 4. The primary gas moverconsisted of diaphragm-type feed air and waste gas compressors driven bya common motor. The weight of each variable-weight component, i.e., theadsorbent, primary gas mover, and battery, were calculated using themethods described earlier for values of the minimum bed pressure between0.35 and 0.96 atma. These weights were summed and all data were plottedas shown in FIG. 11. The three individual component weights show noobvious minima as functions of the minimum bed pressure. When theseweights are combined, however, the plot of total variable weight vs.minimum bed pressure exhibits a minimum total variable weight of 2.7 lbat 0.7 atma. A desirable relative weight range between the minimumweight and 5% above the minimum weight was defined to yield a desirabletotal variable weight range of 2.7 to 2.9 lb. This corresponds to arange of the minimum bed pressure of about 0.5 to 0.9 atma, which is adesirable PVSA operating range for this Example. In this desirablepressure range, the weight of the adsorbent is between a lower value of0.7 lb and an upper value of 1.3 lb, the weight of the primary gas moveris 1.1 lb, and the weight of the battery is between a lower value of 0.5lb and an upper value of 1.0 lb.

EXAMPLE 11

Example 3 was repeated except that the PVSA system was a four bed systemoperated according to the cycle of Tables 3 and 4. The primary gas moverconsisted of scroll-type feed air and waste gas compressors driven by acommon motor. The weight of each variable-weight component, i.e., theadsorbent, primary gas mover, and battery, were calculated using themethods described earlier for values of the minimum bed pressure between0.4 and 1.0 atma. These weights were summed and all data were plotted asshown in FIG. 12. The three individual component weights show no obviousminima as functions of the minimum bed pressure. When these weights arecombined, however, the plot of total variable weight vs. minimum bedpressure exhibits a minimum total variable weight of 2.6 lb at 0.8 atma.A desirable relative weight range between the minimum weight and 5%above the minimum weight was defined to yield a desirable total variableweight range of 2.6 to 2.8 lb. This corresponds to a range of theminimum bed pressure of 0.6 to about 0.9 atma, which is a desirable PVSAoperating range for this Example. In this desirable pressure range, theweight of the adsorbent is between a lower value of 0.6 lb and an uppervalue of 1.1 lb, the weight of the primary gas mover is 1.1 lb, and theweight of the battery is between a lower value of 0.5 lb and an uppervalue of 1.1 lb.

EXAMPLE 12

Example 4 was repeated except that the PVSA system was a four bed systemoperated according to the cycle of Tables 3 and 4. The primary gas moverconsisted of diaphragm-type feed air and waste gas compressors driven bya common motor. The weight of each variable-weight component, i.e., theadsorbent, primary gas mover, and battery, were calculated using themethods described earlier for values of the minimum bed pressure between0.4 and 0.96 atma. These weights were summed and all data were plottedas shown in FIG. 13. The three individual component weights show noobvious minima as functions of the minimum bed pressure. When theseweights are combined, however, the plot of total variable weight vs.minimum bed pressure exhibits a minimum total variable weight of 2.3 lbat about 0.8 atma. A desirable relative weight range between the minimumweight and 5% above the minimum weight was defined to yield a desirabletotal variable weight range of 2.3 to 2.4 lb. This corresponds to arange of the minimum bed pressure of about 0.6 to about 0.9 atma, whichis a desirable PVSA operating range for this Example. In this desirablepressure range, the weight of the adsorbent is between a lower value of0.6 lb and an upper value of 1.1 lb, the weight of the primary gas moveris 0.7 lb, and the weight of the battery is between a lower value of 0.5lb and an upper value of 1.0 lb.

EXAMPLE 13

Example 5 was repeated except that the PVSA system was a four bed systemoperated according to the cycle of Tables 3 and 4. The primary gas moverconsisted of scroll-type feed air and waste gas compressors driven by acommon motor. The weight of each variable-weight component, i.e., theadsorbent, primary gas mover, and battery, were calculated using themethods described earlier for values of the minimum bed pressure between0.35 and 0.96 atma. These weights were summed and all data were plottedas shown in FIG. 14. The three individual component weights show noobvious minima as functions of the minimum bed pressure. When theseweights are combined, however, the plot of total variable weight vs.minimum bed pressure exhibits a minimum total variable weight of 1.4 lbat about 0.7 atma. A desirable relative weight range between the minimumweight and 5% above the minimum weight was defined to yield a desirabletotal variable weight range of 1.4 to 1.5 lb. This corresponds to arange of the minimum bed pressure of 0.6 to 0.9 atma, which is adesirable PVSA operating range for this Example. In this desirablepressure range, the weight of the adsorbent is between a lower value of0.3 lb and an upper value of 0.6 lb, the weight of the primary gas moveris 0.6 lb, and the weight of the battery is between a lower value of 0.3lb and an upper value of 0.5 lb.

EXAMPLE 14

Example 6 was repeated except that the PVSA system was a four bed systemoperated according to the cycle of Tables 3 and 4. The primary gas moverconsisted of diaphragm-type feed air and waste gas compressors driven bya common motor. The weight of each variable-weight component, i.e., theadsorbent, primary gas mover, and battery, were calculated using themethods described earlier for values of the minimum bed pressure between0.4 and 0.96 atma. These weights were summed and all data were plottedas shown in FIG. 15. The three individual component weights show noobvious minima as functions of the minimum bed pressure. When theseweights are combined, however, the plot of total variable weight vs.minimum bed pressure exhibits a minimum total variable weight of 1.2 lbin a range of the minimum bed pressure of about 0.6 to 0.8 atma, whichis a desirable PVSA operating range for this Example. In this desirablepressure range, the weight of the adsorbent is between a lower value of0.3 lb and an upper value of 0.6 lb, the weight of the primary gas moveris 0.4 lb, and the weight of the battery is between a lower value of 0.3lb and an upper value of 0.5 lb.

EXAMPLE 15

Example 7 was repeated except that the PVSA system was a four bed systemoperated according to the cycle of Tables 3 and 4. The primary gas moverconsisted of scroll-type feed air and waste gas compressors driven by acommon motor. The weight of each variable-weight component, i.e., theadsorbent, primary gas mover, and battery, were calculated using themethods described earlier for values of the minimum bed pressure between0.5 and 1.06 atma. These weights were summed and all data were plottedas shown in FIG. 16. The three individual component weights show noobvious minima as functions of the minimum bed pressure. When theseweights are combined, however, the plot of total variable weight vs.minimum bed pressure exhibits a minimum total variable weight of 4.4 lbat slightly below 1.0 atma. A desirable relative weight range betweenthe minimum weight and 5% above the minimum weight was defined to yielda desirable total variable weight range of 4.4 to 4.6 lb. Thiscorresponds to a range of the minimum bed pressure of about 0.8 to 1.1atma, which is a desirable PVSA operating range for this Example. Inthis desirable pressure range, the weight of the adsorbent is between alower value of 1.0 lb and an upper value of 1.6 lb, the weight of theprimary gas mover is 1.6 lb, and the weight of the battery is between alower value of 1.4 lb and an upper value of 2.0 lb.

EXAMPLE 16

Example 8 was repeated except that the PVSA system was a four bed systemoperated according to the cycle of Tables 3 and 4. The primary gas moverconsisted of diaphragm-type feed air and waste gas compressors driven bya common motor. The weight of each variable-weight component, i.e., theadsorbent, primary gas mover, and battery, were calculated using themethods described earlier for values of the minimum bed pressure betweenabout 0.37 and 1.06 atma. These weights were summed and all data wereplotted as shown in FIG. 17. The three individual component weights showno obvious minima as functions of the minimum bed pressure. When theseweights are combined, however, the plot of total variable weight vs.minimum bed pressure exhibits a minimum total variable weight of 3.9 lbat slightly less than 1.0 atma. A desirable relative weight rangebetween the minimum weight and 5% above the minimum weight was definedto yield a desirable total variable weight range of 3.9 to 4.1. Thiscorresponds to a range of the minimum bed pressure of about 0.8 to 1.0atma, which is a desirable PVSA operating range for this Example. Inthis desirable pressure range, the weight of the adsorbent is between alower value of 1.0 lb and an upper value of 1.5 lb, the weight of theprimary gas mover is 1.1 lb, and the weight of the battery is between alower value of 1.4 lb and an upper value of 2.0 lb.

EXAMPLE 17

The PVSA system of FIG. 1 was simulated using the cycle described inTables 1 and 2 for product flow rates of 1 to 3 lpm, product deliverypressures between 1.2 and 1.6 atma, and run times between 1 and 3 hours.The PVSA system of FIG. 1 also was simulated using the cycle describedin Tables 3 and 4 for the same product flow rates, product deliverypressures, and run times. For these simulations, the rechargeablebattery provides between 0.02 and 0.17 KWh of power during the operatingrun time between maximum and minimum working charge. The total workingcapacity of the adsorbent in each adsorber bed during the cycles isbetween 1.2×10⁻⁴ and 6.7×10⁻⁴ lbmoles of nitrogen. The feed aircompressor (the first compressor) moves between 1.14×10⁻⁴ and 4.01×10⁻⁴lbmoles of pressurized feed air during the feed steps and the waste gascompressor (the second compressor) moves between 3.47×10⁻⁴ and 9.96×10⁻⁴lbmoles of waste gas during the depressurization and evacuation steps.

A summary of the results from Examples 1-16 is given in Table 5. Theseresults were utilized to define desirable operating ranges for theweights of the adsorbent, primary gas mover, and battery as functions ofthe product flow rate. This was effected by plotting values of the upperand lower weights corresponding to the upper and lower values of theminimum bed pressure ranges for each of the variable-weight componentsdefined in Examples 1-16 as functions of product flow rates. Linearboundaries to define a desirable operating region in terms of weight vs.product flow rate then were constructed for each component so that allupper and lower values of the minimum weights were included in thisoptimum operating region. In addition, ranges of the minimum weight ofthe battery were normalized to a unit run time and plotted as functionsof the product flow rate to determine an optimum operating region interms of this normalized variable. Based on these Examples, the minimumbed pressure typically falls between 0.25 and 1.0 atma, and may be inthe range of 0.45 and 0.8 atma. TABLE 5 Summary of Examples 1 through 16Product Product Run Minimum Bed Total Variable Example Flow, Press.,Time, No of Primary Gas Pressure, atma Weight, lb No. lpm atma hr BedsMover Type Minimum Range Minimum Range 1 3 1.6 1 5 Scroll 0.7 0.4-0.93.6 3.6-3.8 2 3 1.6 1 5 Diaphragm 0.7 0.5-0.9 2.8 2.8-3.0 3 2 1.4 2 5Scroll 0.7 0.6-0.9 2.9 2.9-3.1 4 2 1.4 2 5 Diaphragm 0.8 0.6-0.9 2.32.3-2.5 5 1 1.2 3 5 Scroll 0.7 0.6-0.8 1.5 1.5-1.6 6 1 1.2 3 5 Diaphragm0.7 0.6-0.8 1.2 1.2-1.3 7 3 1.6 3 5 Scroll 0.9 0.8-1.1 4.8 4.8-5.0 8 31.6 3 5 Diaphragm 0.9 0.8-1.0 4.0 4.0-4.2 9 3 1.6 1 4 Scroll 0.7 0.5-0.93.2 3.2-3.4 10 3 1.6 1 4 Diaphragm 0.7 0.5-0.9 2.7 2.7-2.9 11 2 1.2 2 4Scroll 0.8 0.6-0.9 2.6 2.6-2.8 12 2 1.2 2 4 Diaphragm 0.8 0.6-0.9 2.32.3-2.4 13 1 1.2 3 4 Scroll 0.7 0.6-0.9 1.4 1.4-1.5 14 1 1.2 3 4Diaphragm 0.7 0.6-0.8 1.2 — 15 3 1.6 3 4 Scroll ˜1.0 0.8-1.1 4.4 4.4-4.616 3 1.6 3 4 Diaphragm ˜1.0 0.8-1.0 3.9 3.9-4.1

The resulting plots and desirable operating regions for the individualvariable-weight components are shown in FIGS. 18, 19, 20, and 21. FIG.18 illustrates a desirable operating region bounded by (a) a lower linedrawn through the origin and the lower weight of the adsorbent weightrange for the product flow rate of 3 lpm and (b) an upper line drawnthrough the origin and the upper weight of the adsorbent weight rangefor the product flow rate of 1 lpm. All upper and lower weights of theadsorbent for product flow rates of 1, 2, and 3 lpm thus fall within thedesirable operating region described by the upper and lower lines ofFIG. 18.

FIG. 19 indicates that the desirable range of the weights of the primarygas movers lie between and include the weights of the scroll-type anddiaphragm-type feed air and waste gas compressors which define the upperand lower lines, respectively.

FIG. 20 illustrates a desirable operating region bounded by upper andlower lines drawn through the origin and the upper and lower batteryweights of the Examples at 3 lpm product flow rates. All upper and lowerweights of the variable-weight battery for product flow rates of 1, 2,and 3 lpm thus fall within the desirable operating region described bythe upper and lower lines.

FIG. 21 illustrates a desirable operating region bounded by (a) a lowerline drawn through the origin and the lower value of the time-normalizedbattery weight range corresponding to the Examples at 1 lpm product flowrate and (b) an upper line drawn through the origin and the upper valueof the time-normalized battery weight range corresponding to theExamples at a product flow rate of 3 lpm. All upper and lower values ofthe time-normalized battery weight for product flow rates of 1, 2, and 3lpm thus fall within the desirable operating region described by theupper and lower lines of FIG. 21.

The optimization methods described above thus cover the operation offour bed and five bed PVSA system for production rates of 1 to 3 lpm of93 mole % oxygen in a product pressure range of 1.2 to 1.6 atma forperiods of 1 to 3 hours of continuous run time. The correspondingoptimum weight ranges for individual components were definedanalytically for the desirable operating ranges of the minimum bedpressure. In addition, desirable operating regions were definedanalytically in terms of weight vs. product flow rates for theindividual variable-weight components. Also, desirable operating regionswere defined analytically in terms of weight vs. product flow rates forthe total weights of the combined variable-weight. These are summarizedbelow.

The desirable operating regions described above and illustrated in FIGS.18, 19, 20, and 21 may be expressed as follows for the individualvariable-weight components:

(a) for the weight of the adsorbent, W_(a),0.21 F_(p)<W_(a)<0.61 F_(p);

(b) for the weight of the primary gas mover, W_(p),0.36 F_(p)<W_(p)<0.70 F_(p;)

(c) for the weight of the battery, W_(b),0.18 F_(p)<W_(b)<0.71 F_(p); and

(d) for the battery weight on a time-normalized basis,0.10 F_(p) t_(r)<W_(b)<0.40 F_(p) t_(r).In these expressions, F_(p) is the product flow rate in liters perminute (at 23° C. and 1 atma), weight is in pounds, and time t_(r) is inhours. The desired operating characteristics of the PVSA systemsdescribed above may be characterized by any of the above expressions.

By combining the expressions in (a), (b), and (c) above, the totalvariable weight, W_(t), may be expressed as0.75 F_(p)<W_(t)<2.02 F_(p)where W_(t) is in pounds. The combined weight of the variable-weightcomponents of a PVSA system designed for generating 1 lpm of 93 mole %oxygen thus may lie between 0.75 and 2.02 pounds and a system designedfor generating 3 lpm of 93 mole % oxygen thus may lie between 2.25 and6.06 pounds. This expression may be extended to product flow rates above3 lpm and below 1 lpm to determine the total variable weight of the PVSAsystem components. For example, the expression may be used to determinethe total variable weight between 0.5 and 5 lpm, and this weight wouldrange between 0.375 lb and 1.01 lb for a 0.5 lpm system and between 3.75lb and 10.1 lb for a 5 lpm system.

1. A method for producing an oxygen-rich product gas comprising (a)providing a primary gas mover including a first compressor forcompressing atmospheric air to provide pressurized feed air and a secondcompressor adapted to compress an oxygen-depleted waste gas fromsubatmospheric pressure to atmospheric pressure, a drive motor fordriving the first and second compressors, and a rechargeable battery forproviding power to the drive motor, wherein the rechargeable powersupply is characterized by an operating run time between maximum andminimum working charge; (b) providing a pressure/vacuum swing adsorptionsystem adapted to separate the pressurized feed air into the oxygen-richproduct gas and the oxygen-depleted waste gas, wherein the adsorptionsystem comprises a plurality of adsorber beds containing adsorbent; and(c) operating each of the adsorber beds in turn through an adsorptioncycle including at least the repeating steps of feed/provide product,depressurization, evacuation, and repressurization; wherein the methodis characterized by any of the operating parameters (1) the rechargeablebattery provides between 0.02 and 0.17 KWh of power during the operatingrun time between maximum and minimum working charge; (2) the totalworking capacity of the adsorbent in each adsorber bed during theadsorption cycle is between 1.2×10⁻⁴ and 6.7×10⁻⁴ lbmoles of nitrogen;(3) the first compressor moves between 1.1 4×10⁻⁴ and 4.01 ×10⁻⁴ lbmolesof pressurized feed air during the feed/provide product step; and (4)the second compressor moves between 3.47×10⁻⁴ and 9.96×10⁻⁴ lbmoles ofwaste gas during the depressurization and evacuation steps.
 2. Themethod of claim 1 wherein the pressure/vacuum swing adsorption systemhas four adsorber beds and each of the adsorber beds undergoes in turn aseries of adsorption cycle steps which comprise (A) a feed/make productstep wherein the pressurized feed air is introduced into a feed end ofthe bed while the oxygen-enriched product gas is withdrawn from aproduct end of the bed; (B) a feed/make product/provide repressurizationstep wherein the pressurized feed air is introduced into a feed end ofthe bed while an oxygen-enriched product gas is withdrawn from a productend of the bed, and wherein a portion of the product gas is used forpressurizing another bed undergoing its final repressurization step; (C)a depressurization step in which the bed is depressurized by withdrawinggas therefrom, wherein at least a portion of the gas withdrawn therefromis transferred to another bed undergoing a repressurization step; (D) aprovide purge step in which the bed is further depressurized bywithdrawing gas therefrom, wherein at least a portion of the gaswithdrawn therefrom is transferred to another bed undergoing a purgestep; (E) an evacuation step in which gas is withdrawn from the feed endof the bed until the bed reaches a minimum subatmospheric bed pressure;(F) a purge step in which the bed is purged by introducing purge gasinto the product end of the bed while continuing to evacuate the bed,wherein the purge gas is provided from another bed undergoing step (D);(G) a repressurization step in which pressurization gas is introducedinto the product end of the bed, wherein the pressurization gas isprovided from another bed undergoing step (C); and (H) a finalrepressurization step in which product gas from another bed isintroduced into the product end of the bed.
 3. The method of claim 1wherein the minimum bed pressure is between 0.25 and 1.0 atma.
 4. Themethod of claim 1 wherein the minimum bed pressure is between 0.45 and0.8 atma.
 5. The method of claim 1 wherein the pressure of theoxygen-enriched product gas is between 1.2 and 1.6 atma.
 6. The methodof claim 1 wherein the oxygen-enriched product gas is provided at a flowrate in the range of 0.5 to 3.0 liters per min (defined at 23° C. and 1atma pressure).
 7. A method for producing an oxygen-rich product gascomprising (a) providing a primary gas mover including a firstcompressor for compressing atmospheric air to provide pressurized feedair and a second compressor adapted to compress an oxygen-depleted wastegas from subatmospheric pressure to atmospheric pressure, a drive motorfor driving the first and second compressors, and a rechargeable batteryfor providing power to the drive motor, wherein the rechargeable powersupply is characterized by an operating run time between maximum andminimum working charge; (b) providing a pressure/vacuum swing adsorptionunit adapted to separate the pressurized feed air into the oxygen-richproduct gas and the oxygen-depleted waste gas, wherein the adsorptionunit comprises a plurality of adsorber beds containing adsorbentselective for the adsorption of nitrogen from air; and (c) operatingeach of the adsorber beds in turn through an adsorption cycle includingat least the repeating steps of feed/provide product, depressurization,evacuation, and repressurization; wherein the minimum pressure in theevacuation step is between 0.35 and 1.00 atma.
 8. A method for thedesign of a portable pressure/vacuum swing adsorption oxygenconcentrator system comprising (a) defining design parameters includingat least a product flow rate, a product purity, a product deliverypressure, a pressure/vacuum swing adsorption process cycle, the numberof adsorber vessels, an adsorbent contained in the adsorber vessels, thetype of gas mover, the type of regenerable power supply to provide powerto the drive motor, and the run time of the regenerable power supplybetween maximum and minimum working charge; (b) selecting a series ofminimum bed pressures pressures below atmospheric pressure anddetermining for each of the minimum bed pressures the required weightsof the gas mover, the power supply, and the adsorbent contained in theadsorber vessels, wherein each minimum bed pressure is a lowest bedpressure in the pressure/vacuum swing adsorption process cycle; (c)adding the weights of the adsorbent, the gas mover, and the power supplydetermined in (b) for each value of the minimum bed pressure to providea total weight of the adsorbent, the gas mover, and the power supply asa function of the minimum bed pressure; and (d) selecting a range of theminimum bed pressures that corresponds to a range of minimum combinedweight of the adsorbent, the gas mover, and the power supply.
 9. Themethod of claim 28 wherein the range of minimum bed pressures is between0.45 and 0.8 atma.